1. Field of the Invention
The present invention relates to a plasma processing method and apparatus for processing a target substance contained in gas by using atmospheric-pressure plasma.
2. Description of the Related Art
Air pollution, caused by gas containing volatile compounds and the like, and its impact on the human body have been a focus of attention in recent years. Technologies for decomposing (in particular, detoxifying) gas, such as volatile organic compounds (VOCs), by plasma discharge, especially by nonequilibrium plasma discharge, have been a focus of attention and studied, and methods and apparatuses based on such technologies have been proposed. One of the major factors affecting the decomposition of gas containing volatile compounds and the like is a waveform of applied voltage. Examples of waveforms of applied voltage include waveforms of high-frequency sine-wave alternating voltage, high-frequency square-wave alternating voltage, and, as described in Japanese Patent Laid-Open No. 2000-325735, a waveform generated by superimposing a high-voltage pulse on a commercial-frequency high voltage. These waveforms are applied between electrodes, and cause electrons to vibrate between the electrodes in the direction of the electric field and to collide with gas molecules containing volatile compounds and the like. The gas is thus decomposed by collision ionization. “High frequencies” here mean frequencies ranging from several kHz to several tens of kHz, while a “commercial frequency” means a frequency of 50 or 60 Hz.
However, sufficient processing efficiency typically cannot be achieved, even if the high-frequency sine-wave alternating voltage is used. Moreover, the use of high-frequency square-wave alternating voltage causes heat generation, which prevents high power efficiency from being achieved. Furthermore, when a commercial-frequency power-supply is used, discharge takes place according to a mechanism similar to that of direct voltage. Thus the sparkover voltage increases and a large amount of input power is required.